Lower extremity physical performance, which inherently depends on leg muscle function and on nutritive leg blood flow, serves as a predictor of future disability among independent older adults. Congestive heart failure (CHF) provides a potentially robust model for lower extremity physical performance in the elderly. Several lines of evidence suggest that skeletal muscle and limb blood flow abnormalities that occur during exercise contribute significantly to exercise intolerance in older CHF patients. However, the contribution of skeletal muscle and limb blood flow during exercise to severe exercise intolerance and physical function limitations in older CHF patients is unknown. A major limitation to progress in this area of research has been an inability to noninvasively measure leg blood flow during exercise. Using magnetic resonance imaging (MRI), we recently demonstrated that femoral artery blood flow during rest and after an ischemic stimulus can be measured reproducibly in older subjects. The proposed pilot study would utilize MRI to measure thigh muscle cross-sectional area and femoral artery blood flow at rest and after submaximal exercise in 2 groups of older subjects (15 with CHF and 15 normals) who have markedly different levels of exercise tolerance. We will evaluate the safety, feasibility, and reproducibility of MRI assessments of femoral artery blood flow during submaximal exercise, and we will obtain assessments of variance and correlation of femoral artery blood flow with thigh muscle cross-sectional area, exercise intolerance, strength, and physical function, these measures being needed to estimate effect sizes for a future study. The ability to assess the relative contributions of reduced muscle blood flow and sarcopenia to exercise intolerance has important implications beyond the CHF model, since exercise intolerance contributes significantly to disability in older persons with a variety of disorders.